He was young, only 17, and though there was bravado in his stride as he took the stage, the professional musicians said nothing to him. Watching him seat himself at the piano, they smiled at one another. They knew he had set a trap for himself, and now he would have to pay. They had done this themselves once. Long ago.

The occasion was an unusual benefit concert given by trumpeter Wynton Marsalis and his quintet, who ventured out to Calvin Coolidge High School yesterday to give what one of the 1,000 or so students who attended called "one of the best concerts Coolidge High has ever had." The lecture/concert was sponsored by radio station WDCU-FM (90.1), the D.C. Commission on the Arts and Marsalis himself, who has been giving jazz lectures at inner-city schools, often at his own expense, during stops on his fall tour.

It seemed fitting then, that during a question-and-answer session, a student asked the seven-time Grammy winner and leader of one the most elegant and powerful jazz units currently assembled if he had any openings in his band. Marsalis seemed surprised. "You think you're ready to play?" he asked.

"Yeah."

"C'mon up and play."

Jazz guru Art Blakey once said that jazz is the most honest music in the world, because either you can play it or you can't. This young man who took the stage couldn't play -- not well, anyway -- but before his fellow students could laugh him into oblivion for trying to be a cool guy, Marsalis told a self-deprecating story about his own experience as a young trumpeter.

"When I was a boy," he recalled, "I went to somebody's bandstand, and I sounded so bad ... the person said to me, 'Don't you ever walk on my bandstand -- or any bandstand -- and do that to music.' " The audience of teen-agers, many of whom had rarely been exposed to pure jazz before, listened respectfully as Marsalis told the would-be pianist: "We love you here. I've done what you just did. So don't do that again."

It was an educational afternoon for the hipsters at Coolidge, many of whom said they were go-go music lovers, only slightly familiar with the internationally renowned Marsalis and the music that made him famous. "I know he's a famous jazz trumpet player, but other than that I don't know him that well," said Isaac Norville, 18, before Marsalis' performance. "I hear he practices a lot." Added Michael Edwards, 17, "I don't know much about jazz. I like it so-so ... One of my teachers said to go to one of his concerts cost $25, so you should take advantage of the opportunity to see him."

Actually Marsalis' concert at Blues Alley, where he and his quintet are appearing through next Tuesday, is $15.25 per person plus a $2 drink minimum, but yesterday was a rare opportunity anyway. Marsalis, clad in a fashionable white sports jacket and tie, proved an able lecturer, charming and funny, running through a quick synopsis of the 12-bar blues form and explaining in layman's language what the word "improvisation" meant to a highly skilled jazzman: the range of improvisatory options available, the different styles and techniques used, and so forth. His musicians (Jeff Watts, Marcus Roberts -- recent winner of the Thelonius Monk International Jazz Piano Competition -- Reginald Veal and Todd Williams), who were similarly clad in suits and ties, gave impromptu demonstrations in various playing styles.

The jazzmen were clearly a hit with the young people, who cheered and applauded after solos -- ultimately conducting themselves like a sophisticated jazz audience -- as the Marsalis unit smoked through four bop-like compositions. Indeed, toward the end of the performance -- which became something of a mini-media event after the television cameras showed up and crowded in on the trumpeter -- the Marsalis unit earned the ultimate tribute from a few male members of the Coolidge High student community: the bark. A few scattered howls could be heard mingled with each round of applause. "It's a compliment to him," explained Jackie Millner, 15.

"I hadn't heard of him," she added. "He's a good role model. He looks young and teen-age trumpet players would take after him. He relates well. When he said things about some students not wanting to work or messing around ... you could tell he had been there. I hope his group stays together ... I hope they come back."

Marsalis plans to give a lecture/workshop at Duke Ellington School of the Arts tomorrow and at Lorton Reformatory and Oak Hill Youth Facility in Laurel on Monday and Tuesday of next week (both of those concerts sponsored by the Arts in Prison program of the D.C. Commission on the Arts). "A lot of these kids aren't as fortunate as I was," Marsalis said of his new role as teacher. "All they need to know is somebody loves them but wants something out of them."